NUTRITIONAL REQUIREMENTS OF YEAST 317 



considered by the various authors who have described pathogenic 

 yeasts. See Chapter X. 



NUTRITIONAL REQUIREMENTS OF YEAST 



The yeasts, like other organisms, require adequate supphes of 

 carbon, nitrogen, hydrogen, oxygen, phosphorus, sulphur, magnesium, 

 and other elements. 



Substances like sugars, organic acids, aldehydes, and glycerol can 

 generally be used as sources of carbon for the growth of yeasts. 

 There are some differences, of course, in the abilities of the various 

 yeasts to utilize these substances. The carbon source is generally 

 considered to be of importance in supplying the organisms with 

 energy, i.e., the aerobic or anaerobic breakdown of carbon com- 

 pounds yields energy which can be used by the yeast. Of course, 

 some of the carbon is also used in building protoplasm, together 

 with other elements. 



Degradation products of proteins, such as proteoses, peptones, 

 peptides, amino acids, and ammonia, as well as urea and amides, 

 can, in most instances, serve as sources of nitrogen. Ammonium salts 

 such as sulphate, phosphate, and chloride, are often employed in 

 artificial media used for growing yeasts and are commonly used in 

 the manufacture of compressed yeast. Nitrogen is, of course, a neces- 

 sary component of amino acids which are built up into proteins, an 

 indispensable constituent of protoplasm. 



Obviously, oxygen and hydrogen are necessary, not only for the 

 make-up of protoplasm, but also for the vital functions they per- 

 form in the general economy of the cell. Phosphorus, as phosphates, 

 plays an important role in cell metabolism. The importance of phos- 

 phates will be described later in the discussion of the alcoholic fer- 

 mentation. Phosphorus is also required for the make-up of nucleo- 

 proteins, phospholipids, and the Hke. Sulphur is an essential con- 

 stituent of enzyme systems involving glutathione, thiamin, and so 

 on. Magnesium is required for the operation of certain reactions 

 catalyzed by enzymes (e.g., phosphorylation of glucose). Under 

 certain conditions, manganese and cobalt may be substituted for it. 



In addition to these substances, the yeasts may require other nu- 

 trients, the roles for which are not, in all cases, clearly understood 

 at the present time. Wildiers' observation in 1901 that the addition 

 of a small amount of organic matter, "bios," tremendously stimu- 

 lated the growth of yeast cells in an otherwise chemically defined 

 medium set off a series of far-reaching investigational researches, 



